1. Field of the Invention
The present invention relates to generally to supplying slurry in manufacturing processes and, more particularly, to a method of supplying slurry and a slurry supply apparatus for supplying slurry to a chemical mechanical polishing (CMP) machine used in planarizing surfaces of semiconductor devices during semiconductor fabrication.
2. Description of Related Art
As semiconductor devices become more highly integrated, a planarization technique using a chemical mechanical polishing (CMP) machine is typically employed. The CMP machine typically uses a slurry comprised of an abrasive material and a diluting agent and possibly a curing agent to create a solution that has a polishing selectivity with respect to an underlying layer that acts as a CMP stopping layer. Selection of a particular slurry depends on the semiconductor layers to be palanarized. For example, a silica-based (SiO2) slurry is typically used for planarizing a silicon oxide layer with a silicon nitride layer being used as the CMP stopping layer, while an alumina-based (Al2O3) slurry is widely used for planarizing a copper layer or a tungsten layer. Similarly, a ceria-based (CeO2) slurry may be used for applications where a higher polishing selectivity than that of the silica-based slurry is required.
A conventional slurry supply apparatus for supplying the slurry to the CMP machine comprises a mixing tank wherein undiluted slurry is diluted with de-ionized water. The mixing tank may include one or more sensors for measuring and controlling the composition of the resulting slurry. The conventional slurry supply apparatus may further include a supply tank that stores the diluted slurry created in the mixing tank until it is delivered to a polishing pad of the CMP machine via delivery conduits.
Disadvantageously, unless the slurries are kept in an agitated or suspension state, abrasive particles will precipitate out of the solution and solidify in interconnecting conduits of the delivery system. Further, such precipitated particles flow out of the slurry supply apparatus in an irregular manner, thereby degrading the uniformity of the chemical mechanical polishing process. As a result, it is difficult to supply slurry having a uniform concentration and uniform mixing ratio to the polishing pad and thus difficult to perform a uniform, reliable and stable CMP process.
Conventional approaches to resolving the problems associated with precipitation and coagulation of particulates have been to either: 1) add a re-circulation loop to the conduit delivery system; or 2) combine the separate slurry component solutions as close to a point of use (POU) as possible (i.e. at the polishing pad of the CMP system.) In the re-circulation loop approach, additional conduits are arranged to form a re-circulation loop, wherein the pre-mixed diluted slurry solution is continuously pumped through the system conduits to keep the solution in an agitated or non-precipitating state. As a quantity of solution is required at the polishing pad, it is ported via a dispensing branch or conduit to a dispensing nozzle, and thence to the polishing pad. Alternatively, with the POU system, the separate chemicals are not mixed until a final stage of the supply conduit system, thus requiring no continuous agitation mechanism.
Both of these approaches have significant disadvantages that effect overall CMP system performance and quality. In the pre-mixed re-circulation system, slurry composition cannot be easily modified as system requirements change (i.e. to deliver a polishing abrasive having a different ratio of un-diluted slurry to diluting agent.) In the POU system, delivery of a properly mixed solution at the polishing pad cannot be guaranteed, resulting in a portion of a device being planarized using a very coarse and predominately un-diluted slurry while another portion of the device is planarized solely with the non-abrasive diluting solution. Further, when a curing agent is used, the pre-mixed slurry solution must be delivered and used within a predetermined period of time, before the slurry cures.
To overcome the pre-mixed slurry problem of the re-circulated approach and particularly the time limitations due to the curing additive, the CMP operation would necessarily be halted while the solution either was changed entirely or was re-mixed and re-circulated to attain a new solution mix ratio. In either event, any “old” solution that may remain in the conduits beyond the re-circulation loop toward the polishing pad would have to be removed to avoid contamination at the polishing pad before a new solution has traversed the dispensing conduit and nozzle.
To overcome the non-uniform mixing problem at the dispensing nozzle of the POU approach, the individual slurry component solutions may be combined at a conduit location a further distance back from the dispensing nozzle. This allows more turbulence mixing to occur before the slurry is dispensed. While such a displacement allows for an engineering tradeoff between a POU mix-change flexibility with more uniform mixing prior to the dispensing nozzle, the further away from the dispensing nozzle, the greater the chance of occurrence of the aforementioned changed-solution contamination and precipitation of the particulates in the un-agitated diluted slurry.